The heat of vaporization is the heat that is absorbed to transform a substance from its liquid state to its vapor, that is, to boil or evaporate the liquid substance completely. In common usage, the heat of vaporization is used in place of the more precise term the enthalpy of vaporization, which has the symbol ()Hvap. The enthalpy of vaporization is the heat of vaporization for vaporizing one mole of the substance under three specific conditions: (1) the pressure remains constant, (2) the only possible work that occurs is expansion against the atmosphere (so-called P()V work) and (3) the temperature remains constant during the process. A heat of vaporization for a substance is only valid for conversion of the pure liquid to the pure gaseous state of the substance.
Enthalpies of vaporization can be measured directly for a large number of substances whose natural physical state at experimentally reasonable temperatures is liquid, if the process is carried out sufficiently slowly to avoid temperature changes. Because there is a large change in volume, measurements made in a constant-volume apparatus are not enthalpies and the change in P()V work that is done must be accounted for to obtain the enthalpy for the process.
Enthalpies of vaporization are often specifically designated to show the conditions under which they apply. Unless the vaporization takes place at 298 K (25°C) or one atmosphere pressure, the temperature and pressure are indicated by subscripts: ()Hvap, 0.5 atm for a reaction at one-half atmospheric pressure. The temperature at which a substance vaporizes can be strongly dependent upon the external pressure on the liquid. One atmosphere is the pressure of the atmosphere at sea level, previously defined by the atmospheric pressure that would support a 760 mm column of mercury in a tube closed at the top (a barometer) and now defined in the metric system or Syteme International, SI as 101,325 Pascals where one Pascal is one kilogram meter per seconds squared. Other pressure units that are commonly used are the torr (equal to 1.760 of a standard atmosphere), the Pascal, the bar (equal to 0.9869 atmospheres or 100,000 Pascals) and pounds per square inch (used principally by American engineers). The units typically used for heats of solution are kilojoules per mole or kilocalories per mole.
The enthalpy of vaporization depends on the difference in the energies of attraction of molecules or ions in the liquid state for each other and the energies of attraction for each other in the gaseous state. Since particles in the gaseous state have very small energies of attraction for each other and all molecules or combinations of negative and positive ions have strong attractions for each other, enthalpies of vaporization are always positive. The heat of vaporization of water is unusually large because the strong hydrogenbonding between water molecules in the liquid state. Because a great deal of energy is required to force water molecules apart from each other, water evaporation is an especially effective cooling process.
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